Phase Change Switch with Multi Face Heater Configuration

ABSTRACT

A switching device includes first and second RF terminals disposed over a substrate, one or more strips of phase change material connected between the first and second RF terminals, a region of thermally insulating material that separates the one or more strips of phase change material from the substrate, and a heater structure comprising one or more heating elements that are configured to control a conductive connection between the first and second RF terminals by applying heat to the one or more strips of phase change material. Each of the one or more strips of phase change material includes a first outer face and a second outer face opposite from the first outer face. For each of the one or more strips of phase change material, at least portions of both of the first and second outer faces are disposed against one of the heating elements.

BACKGROUND

Modern electronics applications require switching devices capable ofaccommodating very high frequency signals. For example, fifth generationwireless applications (5G) will operate in frequency bands of 27 GHz(gigahertz) or greater. Maintaining the correct ON/OFF ratio/isolationversus insertion loss/R_(ON) (on-resistance) and C_(OFF)(off-capacitance) is difficult or impossible to achieve in currentsemiconductor switching technologies, such as CMOS technology. Phasechange switches represent one promising alternative technology that canmeet these design requirements for high frequency applications. A phasechange switch utilizes a phase change material to control a conductiveconnection between two terminals. The switching operation is performedby transitioning the phase change material between states, e.g., throughthe application of heat to the phase change material. While promising,phase change switches are in the early stages of development and somedesign challenges are yet to be resolved. For example, device parameterssuch as R_(ON) (on-resistance) C_(OFF) (off-capacitance), powerconsumption, linearity, etc., that are needed in electronicsapplications such as RF-switches for mobile phones are difficult to meetin current phase change switch designs while maintaining acceptabledevice area requirements.

SUMMARY

A switching device is disclosed. According to an embodiment, theswitching device comprises a substrate comprising a main surface, firstand second RF terminals disposed over the main surface and laterallyspaced apart from one another, one or more strips of phase changematerial connected between the first and second RF terminals, a regionof thermally insulating material that separates the one or more stripsof phase change material from the main surface, and a heater structurecomprising one or more heating elements that are configured to control aconductive connection between the first and second RF terminals byapplying heat to the one or more strips of phase change material. Eachof the one or more strips of phase change material comprises a firstouter face and a second outer face opposite from the first outer face.For at least one of the one or more strips of phase change material, atleast portions of both of the first and second outer faces are disposedagainst one of the heating elements.

Separately or in combination, the switching device comprises a pluralityof the strips of phase change material, and wherein sections of the oneor more heating elements are arranged alternatingly between each of thestrips of phase change material.

Separately or in combination, each of the strips of phase changematerial are laterally spaced apart from one another in a lateraldirection that is parallel to the main surface and transverse to acurrent flow direction in the strips of phase change material.

Separately or in combination, the first and second outer faces of eachof the strips of phase change material are oriented substantiallyperpendicular to the main surface, each of the strips of phase changematerial comprise a top outer face that is substantially parallel to themain surface and extends between the first and second outer faces of therespective strip, and a first one of the heating elements wraps aroundthe first outer face, the top outer face, and the second outer face ofeach of the strips of phase change material.

Separately or in combination, the switching device further comprisesfirst and second heating terminals, the first heating element isconnected between the first and second heating terminals, the first andsecond heating terminals are configured to apply the heat by forcingcurrent through the first heating element, and the heater structure isconfigured to force the current through the first heating element in adirection that is transverse to the current flow direction of the phasechange material.

Separately or in combination, each of the strips of phase changematerial are elongated in a vertical direction that is perpendicular tothe main surface.

Separately or in combination, the switching device comprises a region ofheating element material disposed on the region of thermally insulatingmaterial, the region of heating element material comprising an uppersurface that is substantially parallel to the main surface, and aplurality of trenches formed in the upper surface of the region ofheating element material, the strips of phase change material aredisposed within the trenches, and the heater structure comprises aplurality of the heating elements, and each of the heating elements isformed by a section of the heating element material that is disposedbetween two of the trenches.

Separately or in combination, the heater structure is configured toforce current through each of the heating elements in a direction thatis parallel to the current flow direction of the phase change material.

Separately or in combination, the heater structure is configured toforce current through each of the heating elements in a direction thatis transverse to the current flow direction of the phase changematerial.

Separately or in combination, each of the strips of phase changematerial are spaced apart from one another in a vertical direction thatis perpendicular to the main surface, and the heater structure comprisesa plurality of the heating elements arranged alternatingly between thestrips of phase change material in the vertical direction.

Separately or in combination, each of the strips of phase changematerial is electrically connected in series with one another thebetween the first and second RF terminals.

Separately or in combination, each of the strips of phase changematerial is electrically connected in parallel with one another betweenthe between the first and second RF terminals.

Separately or in combination, the first and second outer faces of theone or more strips of phase change material are oriented substantiallyparallel to the main surface, wherein the heater structure comprises afirst heating element that is disposed below the first outer face of theone or more strips of phase change material and a second heating elementthat is disposed above the second outer face of the one or more stripsof phase change material.

Separately or in combination, the first heating element comprises afirst heating element material, and the second heating element comprisesa second heating material that is different from the first heatingelement material.

Separately or in combination, the switching device comprises a pluralityof the strips of phase change material, for a first strip of phasechange material from the plurality at least portions of both of thefirst and second outer faces are disposed against one of the heatingelements, and wherein for a second strip of phase change material fromthe plurality only one of the first and second outer faces are disposedagainst one of the heating elements.

A method of forming a switching device is disclosed. According to anembodiment, the method comprises providing a substrate comprising a mainsurface, forming a layer of thermally insulating material on the mainsurface, forming one or more strips of phase change material on thelayer of thermally insulating material such that the one or more stripsof phase change material are separated from the main surface by a regionof the thermally insulating material, forming first and second RFterminals on the main surface that are laterally spaced apart from oneanother and connected to the one or more strips of phase changematerial, and forming a heater structure comprising one or more heatingelements that are configured to control a conductive connection betweenthe first and second RF terminals by applying heat to the one or morestrips of phase change material. Each of the one or more strips of phasechange material comprises a first outer face and a second outer faceopposite from the first outer face. For at least one of the one or morestrips of phase change material, at least portions of both of the firstand second outer faces are disposed against one of the heating elements.

Separately or in combination, forming the one or more strips of phasechange material comprises forming a plurality of the strips of phasechange material, and forming the heater structure comprises formingsections of the one or more heating elements that are arrangedalternatingly between the strips of phase change material.

Separately or in combination, forming the one or more strips of phasechange material comprises depositing a layer of the phase changematerial on the layer of thermally insulating material and structuringthe layer of the phase change material to form a plurality of fin-shapedsections of the phase change material, and forming the heater structurecomprises conformally depositing a strip of heater element material overthe fin-shaped sections of the phase change material.

Separately or in combination, forming the heater structure comprisesforming a region of the heater element material and forming a pluralityof trenches in the region of heater element material, and forming theone or more strips of phase change material comprises depositing thephase change material within each of the trenches.

Separately or in combination, forming the sections of the one or moreheating elements comprises depositing a plurality of layers of heatingelement material that are stacked on top of one another in a verticaldirection that is perpendicular to the main surface, and forming theplurality of the strips of phase change material comprises depositinglayers of the phase change material alternatingly between the of layersof heating element material.

Separately or in combination, forming the heater structure comprisesdepositing a first heating element material on the substrate beforeforming the one or more strips of phase change material and depositing asecond heating element material on the one or more strips of phasechange material.

BRIEF DESCRIPTION OF THE DRAWINGS

The elements of the drawings are not necessarily to scale relative toeach other. Like reference numerals designate corresponding similarparts. The features of the various illustrated embodiments can becombined unless they exclude each other. Embodiments are depicted in thedrawings and are detailed in the description which follows.

FIG. 1, which includes FIGS. 1A, 1B and 1C, depicts a PCM switchingdevice, according to an embodiment. FIG. 1A depicts a planar view of thedevice, FIG. 1B depicts a cross-sectional view of the device along theplane I-I′ identified in FIG. 1A, and FIG. 1C depicts a cross-sectionalview of the device along the plane II-II′ identified in FIG. 1A.

FIG. 2, which includes FIGS. 2A, 2B and 2C, depicts a PCM switchingdevice, according to an embodiment. FIG. 2A depicts a planar view of thedevice, FIG. 2B depicts a cross-sectional view of the device along theplane I-I′ identified in FIG. 2A, and FIG. 2C depicts a cross-sectionalview of the device along the plane II-II′ identified in FIG. 2A.

FIG. 3, which includes FIGS. 3A and 3B, depicts a PCM switching device,according to an embodiment. FIG. 3A depicts a planar view of the device,and FIG. 3B depicts a cross-sectional view of the device along the planeI-I′ identified in FIG. 3A.

FIG. 4, which includes FIGS. 4A, 4B, and 4C, depicts a technique forforming a PCM switching device, according to an embodiment.

FIG. 5, which includes FIGS. 5A, 5B and 5C, depicts a PCM switchingdevice, according to an embodiment. FIG. 5A depicts a planar view of thedevice, FIG. 5B depicts a cross-sectional view of the device along theplane I-I′ identified in FIG. 5A, and FIG. 5C depicts a cross-sectionalview of the device along the plane II-II′ identified in FIG. 5A.

FIG. 6, which includes FIGS. 6A, 6B and 6C, depicts a technique forforming a PCM switching device, according to an embodiment.

FIG. 7, which includes FIGS. 7A, 7B and 7C, depicts a PCM switchingdevice, according to an embodiment. FIG. 7A depicts a planar view of thedevice, FIG. 7B depicts a cross-sectional view of the device along theplane I-I′ identified in FIG. 7A, and FIG. 7C depicts a cross-sectionalview of the device along the plane II-II′ identified in FIG. 7A.

FIG. 8, which includes FIGS. 8A, 8B and 8C, depicts a PCM switchingdevice, according to an embodiment. FIG. 8A depicts a planar view of thedevice, FIG. 8B depicts a cross-sectional view of the device along theplane I-I′ identified in FIG. 8A, and FIG. 8C depicts a cross-sectionalview of the device along the plane II-II′ identified in FIG. 8A.

DETAILED DESCRIPTION

Embodiments of a PCM (phase change material) switching device aredescribed herein. Advantageously, the PCM switching device includes aheater structure that applies heat to opposite facing outer faces of thephase change material. This multi-faced heating produces a substantiallyhomogenous temperature profile within the phase change material. Thisallows for better control of the phase transition, which leads toimproved performance parameters (e.g. R_(ON), C_(OFF), etc.) for adevice of given areal footprint in comparison to a one-sided heaterconfiguration. In embodiments, the PCM switching device includesmultiple strips of phase change material arranged alternatingly betweenheating elements. This design provides a high-density of the phasechange material within a given area, which enables improved currentcarrying capacity and/or blocking capability. Moreover, in theseembodiments, each strip of phase change material has the advantageousmulti-faced heating structure. Corresponding methods are describedherein to produce these device arrangements with a relatively low numberof processing steps, including steps that are typically implemented insilicon technology.

Referring to FIG. 1, a PCM switching device 100 is depicted, accordingto an embodiment. The PCM switching device 100 includes a substrate 102.Generally speaking, the substrate 102 may include any material that iscompatible with semiconductor processing techniques, e.g., deposition,etching, etc. For example, the substrate 102 may include semiconductormaterials such as silicon (Si), carbon, silicon carbide (SiC), silicongermanium (SiGe), etc. In another example, the substrate 102 includesnon-semiconductor material, e.g., sapphire, glass, diamond, etc. In oneparticular embodiment, the substrate 102 is a commercially availablebulk semiconductor wafer, e.g., a silicon wafer. In another example, thesubstrate 102 is a so-called SOI (Silicon on Insulator) substrate, whichincludes a buried layer of insulating material. The substrate 102includes a main surface 104, which may be a substantially planarsurface.

The PCM switching device 100 includes a region of thermally insulatingmaterial 106 that is formed on the main surface 104 of the substrate102. Generally speaking, the region of thermally insulating material 106can include any thermal insulator that can be formed through typicalsemiconductor processing techniques such as CVD (chemical vapourdeposition). Examples of these thermally insulating materials includeoxides and nitrides, e.g., silicon nitride (SiN), silicon dioxide(SiO₂), silicon oxynitride (SiO_(X)N_(Y)), etc. The region of thermallyinsulating material 106 may include multiple layers of the same ordifferent material.

The PCM switching device 100 further includes first and second RFterminals 108, 110. The first and second RF terminals 108, 110 areelectrically conductive structures that form the input/output terminalsof the PCM switching device 100. The first and second RF terminals 108,110 may include electrically conductive materials, e.g., copper,aluminum, alloys thereof, etc. The first and second RF terminals 108,110 may be configured as externally accessible terminals of the PCMswitching device 100. Alternatively, the first and second RF terminals108, 110 may be lower level terminals that are interconnected to otherdevices and/or externally accessible terminals of the PCM switchingdevice 100 by metallization layers (not shown).

The PCM switching device 100 further includes a strip of phase changematerial 112. The term “strip” means that the phase change material hasan elongated geometry along the main surface 104 of the substrate 102with substantially planar outer faces. In the depicted embodiment, thestrip of phase change material 112 includes top and bottom outer faces114, 116 that are substantially parallel to the main surface 104 of thesubstrate 102, and side outer faces 118 that are substantiallyperpendicular to the main surface 104 of the substrate 102. Thus, thestrip of phase change material 112 has a rectangular cuboidconfiguration. Other cross-sectional geometries are possible.

The phase change material of the strip of phase change material 112 is amaterial that can be transitioned between two different phases that eachhave different electrical conductivity. For example, the phase changematerial may be a material that changes from an amorphous state to acrystalline state based upon the application of heat to the phase changematerial, wherein the phase change material is electrically insulating(i.e., blocks a conductive connection) in the amorphous state and iselectrically conductive (i.e., provides a low-resistance current path)in the crystalline state. Generally speaking, phase change materialshaving this property include chalcogenides and chalcogenide alloys.Specifically, these phase change materials includegermanium-antimony-tellurium (GST), germanium-tellurium, andgermanium-antimony.

The strip of phase change material 112 is connected between the firstand second RF terminals 108, 110. This means that the strip of phasechange material 112 is in low-ohmic contact with both the first andsecond RF terminals 108, 110, either through direct physical contact orby conductive intermediaries. In one example, a conductive material suchas TiN, W, TiPtAu is provided between the first and second RF terminals108, 110 and the phase change material to improve the electricalconnection between the two. When the strip of phase change material 112is in a conductive state, current flows between the first and second RFterminals 108, 110 in a current flow direction 120 of the phase changematerial.

The PCM switching device 100 further includes a heater structure 122.The heater structure 122 includes heating elements that are used to heatthe phase change material. The heating elements include a strip or layerof heating element material that converts electrical energy into heatthrough ohmic heating. Generally speaking, the heating element materialmay include a wide variety of conductive materials including metals suchas copper, aluminum, tantalum, tungsten, nickel, etc. and alloysthereof, and semiconductors (doped or undoped) such as ceramic, silicon,polysilicon, silicon carbide, etc.

In the depicted embodiment, the heater structure 122 includes a firstheating element 124 that is below the strip of phase change material 112and a second heating element 126 that is above the strip of phase changematerial 112. According to an embodiment, the first heating element 124includes a first heating element material and the second heating element126 includes a second heating element material that is different fromthe first heating element material. Generally speaking, the first andsecond heating element materials can be any two different ones of theabove listed conductors and/or semiconductors. In one specific exampleof this embodiment, the first heating element material is polysiliconand the second heating element material is a metal, e.g., TantalumNitride or Tungsten. One benefit of this arrangement is that the firstheating element 124 can be formed by front-end of the line processing,e.g., epitaxy, whereas the second heating element 126 can be formed byback-end of the line processing, e.g., metal deposition or plating.

The PCM switching device 100 further includes first, second, third andfourth heating terminals 128, 130, 132, 134. These heating terminals areelectrically conductive structures that may include electricallyconductive metals such as copper, aluminum, alloys thereof, etc. Thefirst heating element 124 is electrically connected between the firstand second heating terminals 128, 130. Likewise, the second heatingelement 126 is electrically connected between the third and fourthheating terminals 132, 134. The first and second heating terminals 128,130 can be biased to force a current through the first heating element124 such that the first heating element 124 generates heat. Likewise,the third and fourth heating terminals 132, 134 can be biased to force acurrent through the second heating element 126 such that the firstheating element 126 generates heat.

The PCM switching device 100 further includes an insulating liner 136disposed between the heating elements and the strip of phase changematerial 112. The insulating liner 136 is designed to electricallyisolate the heating elements from the strip of phase change material 112while simultaneously permitting substantial heat transfer between thetwo. To this end, the insulating liner 136 may be a relatively thin(e.g., less than 1 μm thick and more typically less than 100 nm thick)layer of dielectric material, e.g., silicon dioxide (SiO₂), siliconnitride (SiN), etc.

The working principle of the PCM switching device 100 is as follows. Theheater structure 122 is configured to control a conductive connectionbetween the first and second RF terminals 108, 110 by applying heat tothe strip of phase change material 112. In an OFF state of the PCMswitching device 100, the phase change material is in an amorphousstate. As a result, the strip of phase change material 112 blocks avoltage applied to the first and second RF terminals 108, 110. In an ONstate of the PCM switching device 100, the phase change material is in acrystalline state. As a result, the strip of phase change material 112provides a low-resistance electrical connection between the first andsecond RF terminals 108, 110. The PCM switching device 100 performs aswitching operation by using the first and second heating elements 124,126 to heat the strip of phase change material 112. The phase changematerial may be transitioned to the amorphous state by applying a shortpulses (e.g., pulses in the range of 50-500 nanoseconds) of highintensity heat which causes the phase change material to reach a meltingtemperature, e.g., in the range of 600° C. to 750° C., followed by arapid cooling of the material. This is referred to as a “reset pulse.”The phase change material may be transitioned to the crystalline stateby applying longer duration pulses (e.g., pulses in the range of 0.5-10microseconds) of lower intensity heat, which causes the phase changematerial to reach a temperature at which the material quicklycrystallizes and is highly conductive, e.g., in the range of 250° C. to350° C. This is referred to as a “set pulse.”

The heater structure 122 of the PCM switching device 100 has anadvantageous multi-face configuration wherein the heating elements aredisposed against two opposite facing outer faces of the strip of phasechange material 112. This configuration mitigates any temperaturegradient in the phase change material during the set and reset pulses.By way of comparison, in a single-face configuration wherein heat isonly applied at one side of the phase change material, the heatdistribution throughout the phase change material can be significantlyasymmetric. Specifically, a single-face configuration may have atemperature gradient of 300° C. or greater between the surface of thephase change material that faces the heating element and the portions ofthe phase change material that are distal to the heating element. Bycontrast, the multi-face configuration of the embodiments describedherein can ensure a temperature gradient of close to zero (e.g., lessthan 50° C.) within the strip of phase change material 112 during theset and reset pulses. As a result, the phase change material can be moreuniformly and rapidly transitioned between the amorphous and crystallinestates. This leads to performance benefits in the PCM switching device100 including lower power consumption, increased linearity, and reducedchip area.

The advantageous multi-face configuration described above includes anyarrangement wherein one of more heating elements are disposed againstfirst and second outer faces of the strip of phase change material 112that are opposite one another. In this context, faces are “opposite” oneanother if they extend transversely to one another and define across-sectional width or thickness of the phase change material. In theembodiment, of FIG. 1, the top and bottom outer faces 114, 116 of thestrip of phase change material 112 are the two opposite faces that areheated by the heater structure 122. Alternatively, the side faces 118 ofthe of the strip of phase change material 112 may be the two oppositefaces that are heated by the heater structure 122. Examples of thesedevices will be illustrated in detail below.

According to an embodiment, in an outer region of the strips of phasechange material 112, only one of the outer faces are disposed againstone of the heating elements. For example, as shown in FIG. 1B, the stripof phase change material 112 includes a central region wherein the firstheating element 124 is disposed against the bottom outer face 114 andthe second heating element 126 is disposed against the top outer face116, and further includes outer regions on either side of a centralregion wherein the first heating element 124 is disposed against thebottom outer face 114 but second heating element 126 is not disposedagainst the top outer face 116.

A heater structure 122 with the advantageous multi-face configurationmay be implemented in a variety of different ways. For example, theheater structure 122 may include multiple heating elements, i.e.,separate spans of heating element material that carry separate currents,as is the case in the embodiment of FIG. 1. Alternatively, the heaterstructure 122 may include a single heating element that is configured toheat two faces of the phase change material by a single current. Thecurrent flow direction of the heater structure 122 may vary. Forexample, in the embodiment of FIG. 1, the heater structure 122 isconfigured to force current through the heating elements in a directionthat is transverse to the current flow direction 120 of the phase changematerial. Alternatively, the heater structure 122 may be configured toforce current through each of the heating elements in a direction thatis parallel to the current flow direction 120 of the phase changematerial. In this arrangement, the heating element terminals may bearranged above or below the first and second RF terminals 108, 110.

Referring to FIG. 2, a PCM switching device 100 is depicted, accordingto another embodiment. In this embodiment, the device includes aplurality of the strips of phase change material 112, with sections of afirst heating element 124 arranged alternatingly between each strip ofphase change material 112. Each strip of phase change material 112 islaterally spaced apart from one another in a lateral direction. Thelateral direction is parallel to the main surface 104 and transverse toa current flow direction 120 of the strips of phase change material 112.The heater structure 122 is configured to force current through thefirst heating element 124 in a direction that is transverse to thecurrent flow direction 120 of the phase change material.

In the PCM switching device 100 of FIG. 2, the advantageous multi-faceconfiguration is obtained by a wrap-around design of the heaterstructure 122. In more detail, each strip of phase change material 112includes first and second side faces 137, 138 that are orientedtransversely (e.g., substantially perpendicular) to the main surface 104of the substrate 102, and a top face 114 that extends between the firstand second side faces 137, 138. The first heating element 124 elementwraps around the first side face 137, the top face 114 and the secondside face 138 of each strip of phase change material 112. Thus, asection of the first heating element 124 is disposed against the firstside face 137, another section of the first heating element 124 isdisposed against the top face 114, and another section of the firstheating element 124 is disposed against the second side face 138. Thisdesign produces a highly uniform temperature distribution throughout thestrip of phase change material 112 during the set and reset pulses, asheat is directly applied to three faces of each strip.

Referring to FIG. 3, an example of a PCM switching device 100 with lowaspect ratio strips of phase change material 112 is depicted, accordingto an embodiment. The PCM switching device 100 includes a plurality ofthe strips of phase change material 112 and sections of heating elementsarranged alternatingly between each of the strips of phase changematerial 112, e.g., in an identical manner as described with referenceto FIG. 2. In this embodiment, the strips of phase change material 112are elongated in a vertical direction (VD) that is perpendicular to themain surface 104. In this context, “elongated in the vertical direction”encompasses any configuration wherein a height of the strip of phasechange material 112 as measured in the vertical direction (VD) isgreater than a width of the strip of phase change material 112 asmeasured in a lateral direction (LD) that is parallel to the mainsurface 104 and perpendicular to the current flow direction 120 of thestrips of phase change material 112. In one specific example, the stripsof phase change material 112 may have a height that is 5-10 timesgreater than their width. In absolute terms, the strips of phase changematerial 112 may have a height in the range of 1-3 μm and a width of0.1-0.5 μm. The low aspect ratio arrangement produces a device with afavorable tradeoff between current carrying capacity and chip area.

Referring to FIG. 4, a technique for forming the PCM switching device100 is depicted, according to an embodiment. Initially, as shown in FIG.4A, the substrate 102 is provided and a layer 202 of thermallyinsulating material is formed on the main surface 104, e.g., usingdeposition. Subsequently, a blanket layer 204 of phase change materialis deposited on the layer 202 of thermally insulating material.Subsequently, as shown in FIG. 4B, the blanket layer 204 of phase changematerial 204 is structured using a patterned mask 206 that is formed onthe blanket layer 204 of the phase change material. The patterned mask206 may include an etch resistant material (e.g., oxide, photomaskmaterial, etc.), and the structuring of the blanket layer 204 may beperformed using known etching techniques. The blanket layer of phasechange material 204 is structured to include trenches 205 that extendcompletely to the layer 202 of thermally insulating material. As aresult, the strips of phase change material 112 are formed by fin shapedstructures corresponding to sections of the blanket layer 204 of phasechange material that are between the trenches 205. The layer 202 ofthermally insulating material forms the region of thermally insulatingmaterial 106 between the strips of phase change material 112 and thesubstrate 102. Subsequently, as shown in FIG. 4C, the patterned mask 206is removed, and the first heating element 124 and the insulating liner136 are formed over the fin shaped strips of phase change material 112.The first heating element 124 and the insulating liner 136 can be formedby a conformal deposition technique, e.g., chemical vapour deposition,wherein the insulating liner 136 is formed on all exposed faces of thephase change material and the region of thermally insulating material106 and the heating element material is formed on the insulating liner136 in a similar manner. As a result, the first heating element 124wraps arounds the fins of phase change material.

Referring to FIG. 5, a PCM switching device 100 is depicted, accordingto another embodiment. Like the previously described embodiments, thePCM switching device 100 of FIG. 5 includes a plurality of the strips ofphase change material 112 and sections of heating elements arrangedalternatingly between each of the strips. In this case, the alternatingarrangement of phase change material and heating elements is provided bytrench structures that are formed in a region 140 of heating elementmaterial. The PCM switching device 100 of FIG. 5 includes a region 140of heating element material that is disposed on the region of thermallyinsulating material 106. A plurality of trenches 142 is formed in anupper surface of the region 140 of heating element material that isparallel to the main surface 104. The phase change material in thetrenches 142 provide the strips of phase change material 112, and theheater structure 122 is provided by sections from the region 140 ofheating element material between the trenches 142.

In the depicted embodiment, the heater structure 122 is configured toforce current through each of the heating elements in a direction thatis parallel to the current flow direction 120 of the phase changematerial. As shown, the heater structure 122 includes an electricalinterconnect structure 144 which contacts sections of the heatingelement material between each of the strips of phase change material112. The electrical interconnect structure 144 may include a combinationof metallization and/or electrical vias. In another embodiment, theheater structure 122 may be configured to force current through each ofthe heating elements in a direction that is parallel to the current flowdirection 120 of the phase change material. This configuration can beobtained by forming the trenches 142 with heating element materialunderneath the trenches 142. As a result, a continuous span of theheating element material can extend across the strips of phase changematerial 112 in the current flow direction 120 of the phase changematerial.

Referring to FIG. 6, a technique for forming the PCM switching device100 is depicted, according to an embodiment. Initially, as shown in FIG.6A, the substrate 102 is provided and a thick layer or region 302 of thethermally insulating material is formed on the main surface 104.Subsequently, a region 140 of heating element material is formed. Asshown in the depicted embodiment, a trench may be formed in the layer orregion 302 of the thermally insulating material, and the region 140 ofheating element material may be formed in this trench. Alternatively,the region 140 of heating element material may be formed as a blanketlayer on top of a thinner layer of thermally insulating material.Subsequently, as shown in FIG. 6B, the region 140 of heating elementmaterial is structured using a patterned mask 304. The patterned mask304 may include an etch resistant material (e.g., oxide, photomaskmaterial, etc.), and the structuring may be performed using knownetching techniques. As a result of the structuring, a plurality oftrenches 142 is formed in the region 140 of heating element materialwith discrete spans of the heating element material disposed on eitherside of the trenches 142. Subsequently, as shown in FIG. 6C, thepatterned mask 304 is removed, and the strips of phase change material112 and the insulating liner 136 are formed in each of the trenches 142.The strips of phase change material 112 and the insulating liner 136 maybe formed by depositing the phase change material in the trenches 142,following by a planarization step (e.g., CMP (chemical-mechanicalpolishing) to remove excess material that forms above the trenches 142.

Referring to FIG. 7, a PCM switching device 100 is depicted, accordingto another embodiment. Like the previously described embodiments, thePCM switching device 100 of FIG. 7 includes a plurality of the strips ofphase change material 112 and sections of heating elements 124 arrangedalternatingly between each of the strips. In this embodiment, each ofthe strips of phase change material 112 are spaced apart from oneanother in a vertical direction (VD) that is perpendicular to the mainsurface 104. The heater structure 122 includes a plurality of heatingelements 124 arranged alternatingly between the strips of phase changematerial 112 in the vertical direction (VD). Thus, the PCM switchingdevice 100 of FIG. 7 is configured as a vertically layered stack ofphase change material and heating elements. In this arrangement, theheating elements 124 are disposed against the top outer face 114 and thebottom outer face 116 of each strip of phase change material 112 andthus enable the advantageous temperature uniformity as described abovefor each strip.

In the PCM switching device 100 of FIG. 7 each of the strips of phasechange material 112 are electrically connected in series between thebetween the first and second RF terminals 108, 110. This seriesconfiguration is made possible by electrically conductive via structures146 that vertically extend through the thermally insulating material 106and form an electrical connection between immediately adjacent strips ofphase change material 112. These via structures 146 can be tungsten orpolysilicon plugs, for example. A lowermost one of the strips of phasechange material 112 is electrically connected to the first RF terminal108, and an uppermost one of the strips of phase change material 112 iselectrically connected to the second RF terminal 110. As a result, thestrips of phase change material 112 form a single current path betweenthe first and second RF terminals 108, 110. The PCM switching device 100of FIG. 7 includes vertical electrical connectors 148 that electricallyconnect the first heating terminal 128 with each of the heating elements124 at one end, and electrically connect the second heating terminal 130with each of the heating elements 124 at an opposite end. These verticalelectrical connectors 148 may include conductive metals or highly dopedpolysilicon, for example.

Referring to FIG. 8, a PCM switching device 100 is depicted, accordingto another embodiment. The embodiment of FIG. 8 is substantiallyidentical to that of FIG. 7, except that each of the strips of phasechange material 112 are electrically connected in parallel between thefirst and second RF terminals 108, 110. The parallel configuration ismade possible by vertical electrical connectors 150 that electricallyconnect the first RF terminal 108 with each strip of phase changematerial 112 and electrically connect the second RF terminal 110 witheach strip of phase change material 112 at an opposite end. Thesevertical electrical connectors 150 may include conductive metals orhighly doped polysilicon, for example.

The vertically stacked configuration of the PCM switching device 100 ineither of the embodiments of FIGS. 7 and 8 offers advantageous spaceefficiency by providing multiple strips of phase change material 112 ontop of one another. While the depicted embodiment shows a layered stackof three strips of phase change material 112, in principle this conceptmay be extended to stacks of four, five, etc. As a result, a PCMswitching device 100 with favorable performance benefits, e.g., voltageblocking, current capacity, on-resistance, etc., can be obtained withina small areal footprint.

The vertically stacked design of the PCM switching devices 100 of FIGS.7 and 8 can be obtained through successive deposition of a layer of theheating element material, a layer of the insulating liner, a layer ofthe phase change material, another layer of the insulating liner, and soforth. The strips of phase change material 112 and heating elements 124can be structured using known techniques.

FIGS. 2-8 illustrate exemplary embodiments of a PCM switching device 100with a plurality of the strips of phase change material 112. In each ofthe depicted examples, the heater structure 122 is configured such thatheating elements are disposed against two outer faces of each strip ofphase change material 122. In other embodiments which include multiplestrips of phase change material 112, the heater structure 122 isconfigured such that heating elements are disposed against two outerfaces of a first strip of phase change material 112 and such thatheating elements are disposed against only one outer face of a secondstrip of phase change material 112. Using FIG. 5C to illustrate thisconcept, an outer one of the trenches 142 may be formed such that thereis no heating element material between an outer sidewall of the trench142 and the thermally insulating material 106. As a result, the strip ofphase change material 112 in the outer trench 142 includes an inner sideface that is against the heating element material and an outer side facethat does not face any heating element material. In this arrangement,the central strip of phase change material 112 includes two side outerfaces that are disposed against the heating element material. Moregenerally, this concept can be applied to any embodiment which includesa plurality of the strips of phase change material 112, including thespecific embodiments shown in FIGS. 2-8.

The term “electrically connected,” “directly electrically connected” andthe like as used herein describes a permanent low-impedance connectionbetween electrically connected elements, for example a direct contactbetween the relevant elements or a low-impedance connection via a metaland/or a highly doped semiconductor.

As used herein, the terms “having,” “containing,” “including,”“comprising” and the like are open-ended terms that indicate thepresence of stated elements or features, but do not preclude additionalelements or features. The articles “a,” “an” and “the” are intended toinclude the plural as well as the singular, unless the context clearlyindicates otherwise.

It is to be understood that the features of the various embodimentsdescribed herein may be combined with each other, unless specificallynoted otherwise.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat a variety of alternate and/or equivalent implementations may besubstituted for the specific embodiments shown and described withoutdeparting from the scope of the present invention. This application isintended to cover any adaptations or variations of the specificembodiments discussed herein. Therefore, it is intended that thisinvention be limited only by the claims and the equivalents thereof.

What is claimed is:
 1. A switching device, comprising: a substratecomprising a main surface; first and second RF terminals disposed overthe main surface and laterally spaced apart from one another; one ormore strips of phase change material connected between the first andsecond RF terminals, a region of thermally insulating material thatseparates the one or more strips of phase change material from the mainsurface; and a heater structure comprising one or more heating elementsthat are configured to control a conductive connection between the firstand second RF terminals by applying heat to the one or more strips ofphase change material, wherein each of the one or more strips of phasechange material comprises a first outer face and a second outer faceopposite from the first outer face, and wherein for at least one of theone or more strips of phase change material, at least portions of bothof the first and second outer faces are disposed against one of theheating elements.
 2. The switching device of claim 1, wherein theswitching device comprises a plurality of the strips of phase changematerial, and wherein sections of the one or more heating elements arearranged alternatingly between each of the strips of phase changematerial.
 3. The switching device of claim 2, wherein each of the stripsof phase change material are laterally spaced apart from one another ina lateral direction that is parallel to the main surface and transverseto a current flow direction in the strips of phase change material. 4.The switching device of claim 3, wherein the first and second outerfaces of each of the strips of phase change material are orientedsubstantially perpendicular to the main surface, wherein each of thestrips of phase change material comprise a top outer face that issubstantially parallel to the main surface and extends between the firstand second outer faces of the respective strip, and wherein a first oneof the heating elements wraps around the first outer face, the top outerface, and the second outer face of each of the strips of phase changematerial.
 5. The switching device of claim 4, further comprising firstand second heating terminals, wherein the first heating element isconnected between the first and second heating terminals, wherein thefirst and second heating terminals are configured to apply the heat byforcing current through the first heating element, and wherein theheater structure is configured to force the current through the firstheating element in a direction that is transverse to the current flowdirection of the phase change material.
 6. The switching device of claim4, wherein each of the strips of phase change material are elongated ina vertical direction that is perpendicular to the main surface.
 7. Theswitching device of claim 3, wherein the switching device comprises: aregion of heating element material disposed on the region of thermallyinsulating material, the region of heating element material comprisingan upper surface that is substantially parallel to the main surface; anda plurality of trenches formed in the upper surface of the region ofheating element material, wherein the strips of phase change materialare disposed within the trenches, wherein the heater structure comprisesa plurality of the heating elements, and wherein each of the heatingelements is formed by a section of the heating element material that isdisposed between two of the trenches.
 8. The switching device of claim7, wherein the heater structure is configured to force current througheach of the heating elements in a direction that is parallel to thecurrent flow direction of the phase change material.
 9. The switchingdevice of claim 7, wherein the heater structure is configured to forcecurrent through each of the heating elements in a direction that istransverse to the current flow direction of the phase change material.10. The switching device of claim 2, wherein each of the strips of phasechange material are spaced apart from one another in a verticaldirection that is perpendicular to the main surface, and wherein theheater structure comprises a plurality of the heating elements arrangedalternatingly between the strips of phase change material in thevertical direction.
 11. The switching device of claim 10, wherein eachof the strips of phase change material is electrically connected inseries with one another the between the first and second RF terminals.12. The switching device of claim 10, wherein each of the strips ofphase change material is electrically connected in parallel with oneanother between the between the first and second RF terminals.
 13. Theswitching device of claim 1, wherein the first and second outer faces ofthe one or more strips of phase change material are orientedsubstantially parallel to the main surface, wherein the heater structurecomprises a first heating element that is disposed below the first outerface of the one or more strips of phase change material and a secondheating element that is disposed above the second outer face of the oneor more strips of phase change material.
 14. The switching device ofclaim 13, wherein the first heating element comprises a first heatingelement material, and wherein the second heating element comprises asecond heating material that is different from the first heating elementmaterial.
 15. The switching device of claim 1, wherein the switchingdevice comprises a plurality of the strips of phase change material,wherein for a first strip of phase change material from the plurality atleast portions of both of the first and second outer faces are disposedagainst one of the heating elements, and wherein for a second strip ofphase change material from the plurality only one of the first andsecond outer faces are disposed against one of the heating elements. 16.A method of forming a switching device, the method comprising: providinga substrate comprising a main surface; forming a layer of thermallyinsulating material on the main surface; forming one or more strips ofphase change material on the layer of thermally insulating material suchthat the one or more strips of phase change material are separated fromthe main surface by a region of the thermally insulating material;forming first and second RF terminals on the main surface that arelaterally spaced apart from one another and connected to the one or morestrips of phase change material; and forming a heater structurecomprising one or more heating elements that are configured to control aconductive connection between the first and second RF terminals byapplying heat to the one or more strips of phase change material,wherein each of the one or more strips of phase change materialcomprises a first outer face and a second outer face opposite from thefirst outer face, and wherein for at least one of the one or more stripsof phase change material, at least portions of both of the first andsecond outer faces are disposed against one of the heating elements. 17.The method of claim 16, wherein forming the one or more strips of phasechange material comprises forming a plurality of the strips of phasechange material, and wherein forming the heater structure comprisesforming sections of the one or more heating elements that are arrangedalternatingly between the strips of phase change material.
 18. Themethod of claim 17, wherein forming the one or more strips of phasechange material comprises depositing a layer of the phase changematerial on the layer of thermally insulating material and structuringthe layer of the phase change material to form a plurality of fin-shapedsections of the phase change material, and wherein forming the heaterstructure comprises conformally depositing a strip of heater elementmaterial over the fin-shaped sections of the phase change material. 19.The method of claim 17, wherein forming the heater structure comprisesforming a region of the heater element material and forming a pluralityof trenches in the region of heater element material, and whereinforming the one or more strips of phase change material comprisesdepositing the phase change material within each of the trenches. 20.The method of claim 17, wherein forming the sections of the one or moreheating elements comprises depositing a plurality of layers of heatingelement material that are stacked on top of one another in a verticaldirection that is perpendicular to the main surface, and wherein formingthe plurality of the strips of phase change material comprisesdepositing layers of the phase change material alternatingly between theof layers of heating element material.